The present invention relates to a high voltage spiral-shaped diffused resistor integrated in a semiconductor substrate and liable to have one of its extremities connected to a point of the substrate submitted to high voltage and the other extremity submitted to low voltage.
Such a resistor is described in U.S. Pat. No. 4,792,840. This resistor is constituted by a spiral-shaped region of a second conductivity type and having a given doping level, formed on a first surface of a semiconductor substrate of the first conductivity type and comprises a first terminal associated with a first extremity of the spiral and a second terminal on a second surface of the substrate, the second extremity of the spiral being connected to an overdoped region of the same conductivity type as the substrate.
It will be clear to those skilled in the art that such a resistor can be formed by connecting an extremity of the spiral with the rear surface of the substrate either, as this is more particularly described in the above U.S. patent, at the center of the spiral or at the external spiral extremity. The second method will be preferably chosen in case the spiral surrounds the semiconductpr chip on which it is arranged, so that the spiral further serves as peripheral guard rings.
However, the realization of a spiral resistor such as described in the above U.S. patent causes implementation difficulties especially as regards breakdown voltage and adjustment of the resistor value.
Indeed, the number of spiral turns and the distance between each turn have to be selected as a function of electrostatic requirements on the basis of the maximum difference value of the voltage liable to be applied between the front and rear surfaces of the substrate, while taking into account the doping values of the substrate and spiral. However, when considering a spiral such as illustrated in FIG. 2 of the abovementioned U.S. patent, and comprising a sprial winding around a central point, it can be seen that the last turn of the spiral has a length much larger than that of the preceding turns, especially of the inner turn. Thus, the voltage drop on the last turn will be much higher than the voltage on each of the other turns. As a result, it is not possible for tightly rolled up spirals to exceed a given breakdown voltage for determined values of the various doping levels.
Moreover, in the above U.S. patent, the spiral resistor is formed as part of an integrated circuit comprising other components and more particularly power bipolar or MOS transistors. If it is desired to form a low cost spiral, it has to be formed during the same manufacturing steps as circuits arranged on the same integrated circuit wafer, that is, its doping level will be chosen among the doping levels already provided for the manufacturing of the integrated circuit referred to. For example, in some integrated circuits there are only two steps for doping a substrate according to the conductivity type opposite to that of the substrate, a so-called low doping level and a so-called high doping level. It has been indicated in the above that the number of turns to be provided for the spiral depends upon electrostatic parameters associated with the voltage it has to withstand; therefore, for obtaining a predetermined resistance, it should be possible to modify its resistivity by unit length, that is, on its section or on its doping level. It is not economical to act on the doping level or on the depth of the diffused layer used by the spiral because this would require an additional manufacturing step, and also, it is not convenient to act on the spiral width because this would entail excessive bulkiness thereof.
Thus, an object of the invention is to improve the realization of spiral resistors of the above type to avoid the two above mentioned drawbacks, that is, to permit, on the one hand, to increase the maximum breakdown voltage of a spiral resistor and, on the other hand, to form spiral resistors having predetermined values, without increasing the number of manufacturing steps for the integrated circuit.